Nerve conduction studies test the function of motor and sensory nerves by measuring nerve conduction velocity. Small electrical stimuli are applied to nerves while recordings are made from muscles. Abnormalities may indicate conditions like peripheral neuropathy or radiculopathy. The test evaluates nerves like the median and ulnar nerves and can help diagnose disorders affecting the peripheral nervous system.

1. NERVE
CONDUCTION
STUDY
Garaka Rabel

2. Nerve Conduction Study
(NCS)
• NCS is a test commonly used to
evaluate the function of the motor
and sensory nerves of the human
body.
• Nerve conduction velocity (NCV) is a
common measurement made during
this test.
• The term NCV often is used to mean the actual
test, but this may be misleading since, velocity is
only one measurement in the test suite.

3. Uses
• Nerve conduction studies are used mainly for
evaluation of paresthesias (numbness,
tingling, burning) and/or weakness of the
arms and legs.
• The type of study required is dependent in
part, by the symptoms presented.
• Some indications of nerve conduction studies
are:
– Symptoms indicative of nerve damage as numbness,
weakness.
– Differentiation between local or diffuse disease process
(mononeuropathy or polyneuropathy).
– Get prognostic information on the type and extent of nerve
injury.

4. Common disorders diagnosed by NCS
Peripheral neuropathy
• Mononeuropathy (ex: carpal tunnel syndrome)
• Mononeuritis multiplex (ex: vasculitides, rheumatoid arthritis, lupus
erythematosus [SLE], sarcoidosis, leprosy, Lyme disease, amyloidosis)
• Polyneuropathy (ex: diabetic neuropathy,)
Myopathy
• Muscular dystrophies (ex: Facioscapulohumeral muscular dystrophy)
• Myotonia
• Congenital myopathies
• Metabolic myopathies
Radiculopathy (problem in which one or more nerves are
affected with emphasis on the nerve root; Radix = "root")
• Nerve damage from herniated discs
Diseases of neuromuscular junction
• Myasthenia gravis

5. NERVE CONDUCTION STUDY
PROCEDURE

6. Description of the procedure
Electrodes
• Skin will be cleaned
• electrodes will be taped to the skin along the nerves
that are being studied
Stimulus
• Small stimulus is applied (electric current) that
activate nerves
Current
• The electrodes will measure the current that travels
down the nerve pathway

7. Description of the procedure
(continued..)
If damaged?
• If the nerve damaged, the current will be slower and
weaker
Time
• The procedure takes about 30-90 minutes
Complications
• No reported complication from the procedure
• expect feeling discomfort from electrical current, but
not painful

8. Important points about NCS
• The test is not invasive.
• No contraindication to the procedure,
but if there is an artificial pacemaker,
appropriate precautions should be
taken.
• Anesthesia is not used for this
procedure.
• No special post procedure precautions.
• The test is sometimes combined with
Electromyography (EMG).

9. Components of NCS
• The NCS consists of the following
components:
– Compound Motor Action Potential (CMAP);
also called Motor nerve conduction study
– Sensory Nerve Action Potential (SNAP);
also called Sensory nerve conduction study
– F-wave study
– H-reflex study
– A-(Axon) wave study
will not be
– Blink Reflex study discussed…
– Direct Facial Nerve Study

10. Motor nerve conduction study
• This NCS represents the conduction of an impulse along
peripheral motor nerve fibers.
• It is recorded as a compound evoked potential from a
motor point within the muscle.
• The time it takes for electrical impulse to travel from the
stimulation to the recording site is measured.
• This value called latency and measured in milliseconds
(ms).
• The size of the response called the amplitude and
measured in millivolts (mv).
• By stimulating in two or more different locations along
the same nerve, NCV across different segments can be
measured.

11. Motor nerve conduction study (cont..)
• It corresponds to the integrity of the motor unit but cannot
distinguish between pre- and postganglionic lesions because the
cell body is located in the spinal cord.
• It can be abnormal with normal SNAPs if the lesion is proximal to
the DRG or affecting a purely motor nerve.
• The active and reference pickup should not be too close together.
If this occurs, similar waveforms are recorded at both sites and
rejected, dropping the amplitude of the waveform
effect on the amplitude of varying
the inter-electrode separation.
I: Normal.
Compound Motor Action Potential II: Pickups are too close.

12. Motor nerve conduction study – sites
Median nerves (R & L) at;
• Wrist Abductor Pollicis Brevis
• Elbow
Ulnar nerves (R & L) at;
• Wrist First Dorsal Interosseous (FDI)
• Elbow Abductor Digiti Minimi (ADM)
Peroneal nerves (R & L) at;
• Ankle Extensor Digitorum Brevis
• Head of fibula Tibialis Anterior
Tibial nerves(R & L) at;
• Ankle Abductor Hallucis
Abductor Digiti Quinti Pedis

13. Sensory nerve conduction study
• This NCS represents the conduction of an impulse along the
sensory nerve fibers.
• It is performed by electrical stimulation of a peripheral nerve
and recording from a purely sensory portion of the nerve,
such as on a finger.
• The recording electrode is placed proximal to the stimulating
electrode. (antidromic nerve impulse is recorded)
• Like the motor studies, sensory latencies are on the scale of
milliseconds (ms).
• Sensory amplitudes are much smaller than the motor
amplitudes, usually in the microvolt (μV) range.
• The sensory NCV is calculated based upon the latency and the
distance between the stimulating and recording electrode.

14. Sensory nerve conduction study (cont..)
• It can also be useful in localizing a lesion in relation to the dorsal
root ganglion (DRG).
• The DRG is located in the neural foramen and contains the
sensory cell body. Lesions proximal to it (root, spinal cord)
preserve the SNAP despite clinical sensory abnormalities.
• This is because axonal transport from the cell body to the axon
continues to remain intact.
• SNAPs are typically considered more sensitive than CMAPs in the
detection of an incomplete peripheral nerve injury.
• Antidromic Studies;
– Are easier to record a response than orthodromic studies
– Are less uncomfortable when orthodromic studies secondary to less
stimulation required
– Have larger amplitudes due to the nerve being more superficial at the
distal recording sites

16. Sensory nerve conduction study – sites
Median nerves (R & L) at;
• index finger
• thumb
Ulnar nerves (R & L) at;
• little finger
• ring finger
Sural nerves (R & L) at;
• behind the Lateral Malleolus
Saphenous nerves(R & L) at;
• anterior to the Medial Malleolus

17. F-wave study
• This NCS evokes a small late response from a short duration
supramaximal stimulation.
• It initiates an antidromic motor response to the spinal cord followed
by an orthodromic motor response to the recording electrode.
• It is approximately 5% of the compound motor action potential
(CMAP) height.
• The configuration and latency change with each stimulation.
• This is due to a polysynaptic response in the spinal cord, where
Renshaw cells (R) inhibit impulses from traveling the same path
each time.

18. F-wave study (continued..)
• This is not a reflex, because action potentials travels from the site
of the stimulating electrode in a limb to the spinal cord and back to
the limb in the same nerve that was stimulated.
• The F- waves latency can be used to derive the conduction
velocity of nerves between the limb and spinal cord, whereas the
motor and sensory nerve conduction study in the same segment of
the limb.
• Conduction velocity is derived by measuring the limb length in
millimeters from the stimulation site to the corresponding spinal
segment (ex: C7 spinous process to wrist crease for median nerve).
• This is multiplied by 2 as it goes to the cord and returns to the
muscle.
• Limitation: This evaluates a long neural pathway, which can dilute
focal lesions and hinder specificity of injury location. It only
accesses the motor fibers.

19. H- reflex study (continued..)
• This NCS creates a late response that is an electrically evoked
analogue to a monosynaptic reflex.
• It is initiated with a submaximal stimulus at a long duration
(0.5–1.0 milliseconds).
• This preferentially activates the IA afferent nerve fibers,
causing an orthodromic sensory response to the spinal cord,
and then an orthodromic motor response back to the
recording electrode.
• The morphology and latency remains constant with each
stimulation at the appropriate intensity.

20. H- reflex study
• This NCS creates a late response that is an electrically evoked
analogue to a monosynaptic reflex.
• It is initiated with a submaximal stimulus at a long duration
(0.5–1.0 milliseconds).
• This preferentially activates the IA afferent nerve fibers,
causing an orthodromic sensory response to the spinal cord,
and then an orthodromic motor response back to the
recording electrode.
• The morphology of wave pattern and latency remains
constant with each stimulation at the appropriate intensity.

21. How they are monitored…

22. Interpretation of nerve conductions
• The speed of nerve conduction is related to
– the diameter of the nerve and,
– the degree of myelination (a myelin sheath is a type of "insulation"
around the nerve).
• A normally functioning nerve will transmit a stronger and
faster signal than a damaged nerve.
• In general, the range of normal conduction velocity will be
approximately 50 to 60 meters per second. However, the
normal conduction velocity may vary from one individual to
another and from one nerve to another.
• The Interpretation of nerve conductions is complex, but in
general, different pathological processes result in:
– changes in the latencies
– changes in the amplitudes
– slowing of the conduction velocity

23. Interpretation of nerve conductions
(continued..)
• Examples;
– slowing of the NCS usually indicates there is
damage to myelin.
– slowing across the wrist for the motor and sensory
latencies of the median nerve indicates focal
compression of the median nerve at the wrist,
called carpal tunnel syndrome.
– slowing of all nerve conductions in more than one
limb indicates generalized peripheral neuropathy
(eg. in diabetes mellitus).

24. References
• National Center for Biotechnology Information
(NCBI) web site (26.06.2011)
http://www.ncbi.nlm.nih.gov/books/NBK2
7199/#A7198
• Wikipedia, the free encyclopaedia
(26.06.2011)
http://en.wikipedia.org/wiki/Nerve_condu
ction_study

25. Special Thanks!
• Dr. Sudath Gunasekera,
Consultant Clinical Neurophysiologist,
NHSL.